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Measuring pH in organic solvents is challenging. When organic solvents are present in appreciable quantities, there is a shift in pH readings due to the effects of non-aqueous solvents on the activity of hydrogen ions and on the pH electrode. This can ultimately lead to variable readings and longer response times, obtaining inaccurate and irreproducible results.
Moreover, conventional pH sensors are not designed for such applications. For instance, the aqueous electrolyte of conventional pH electrodes, like KCl solution, may not be miscible or may not dissolve into the sample being tested. In such cases, fluctuations in the readings may occur. In addition, crystallization of KCl may take place, clogging the junction and preventing the outflow of electrolyte, and therefore causing unstable readings. Choosing the right sensor is essential in obtaining accurate results.
METTLER TOLEDO's InLab® Science Pro-ISM pH sensor is a suitable electrode for such non-aqueous applications. The movable sleeve junction ensures easy and adequate outflow of reference electrolyte into the sample and is easy to clean in case of any blockage. This sensor has an added advantage of using two electrolytes of which the outer bridge electrolyte can be changed from a conventional 3 mol/L KCl solution to 1 mol/L LiCl in an ethanol solution or any other electrolyte as needed. These factors contribute towards obtaining stable readings and give reliable pH results.
These are only some of the benefits offered by this sensor specialized in measuring the pH in organic solvents. To learn more about the advantages of using this sensor, read this white paper. It also provides guidance on how to achieve accurate results, for both water-miscible and non-water-miscible organic solvents, respectively.
Why is measuring the pH in organic solvents challenging?
Conventional pH scale of 0 -14 is not applicable due to change in sample dissociation pattern in a respective solvent. Also, organic solvents are usually ion deficient and therefore the measurements are often unstable and require longer time to stabilize. The aqueous outer electrolyte (3M KCl) is not suitable for use in an organic medium and must be replaced with an ethanolic solution of LiCl to obtain stable readings. Such samples require additional understanding to perform the pH measurement and sensor selection is key to obtain results with improved accuracy.
Where is the pH of organic solvents measured and why is it important?
Several industries like paints and pigments, inks, oils, petroleum and petrochemicals, pharmaceuticals, biofuels etc. involve measuring pH of their samples, dispensed / dissolved in organic solvents, at frequent interval as a part of their routine measurements.
How can I measure the pH of organic solvents?
Organic solvents are classified as water miscible and water immiscible solvents. Therefore, pH measurements in non-aqueous samples, can be categorized into two sections:
- The pH measurement of water miscible solvents, which is conducted by immersing the electrode in the solvent or its mixture (with water)
- The pH measurement of water immiscible organic solvents, which is conducted by performing a solvent extraction of the sample in the aqueous phase with deionized water, followed by the pH measurement
